How much do aerodynamics truly benefit the average bent rider?

I'm contemplating another bent purchase, drawn to the simple efficiency and comfort of a high racer. I know I'm always hovering around 13-17 mph on my steel touring bikes, sure I'll pick up a bit on a high racer at some point. I've often heard and read that after 18 mph or so, most effort is burned overcoming aerodynamics. If this is true, what true benefits might the average bent rider gain from a slippery profile and high bottom bracket, if we're mostly staying under 18 mph? I'd love to hear some real world experience about this.

I might buy the bike anyways for it's comfort, and that I'm drawn to it, but should the aero factor even be considered? I know the answer is yes bombing a hill, or stuck in headwinds, what about the other 80% of basic pedaling?

Aerodynamics being what they are, if you ride at 13 mph and get a 'regular' 'bent, you won't see much difference at all. If you get a highracer, you might see an extra mph on flat ground, which could easily be erased by any hills. But the thing with aerodynamics is, the faster you go, the more you benefit. It's a curve, not a stair-step. So depending on your psychology, you may find that you're pushing yourself harder to get more benefit, in which case you could end up at 2-6 mph or more faster.

On a relatively upright trike, I don't get much (if any) aero benefit over a typical upright touring bike in still air. With a quartering headwind, I often find myself in a wind shadow due to roadside vegetation or guardrails. Windspeed also drops off the closer it is measured to the ground, giving a further advantage.

I added a small quarter fairing to my Harley Davidson Roadster, An EXTREME difference was felt by lessening, practically eliminating the hurricane force wind load on my chest.

It was extremely effective above 45 mph, made passing big trucks, cutting through their wind field much better, far less turbulence.

As for low speed, my estimate was right about at 20 mph I could feel the effect and advantage and In almost any headwinds it was nice.
BUT that's a near 600 pound bike plus my 200 in riding gear weight so,
yeah I'd think you'd like one.
Many have them....

I'm rarely above 17,18 on my LWB Recumbent so I see no benefit. But Every time I get into head winds I think about a fairing...

I read somewhere that the high or low racer vs. standard upright bikes is about 1/3rd the wind resistance give or take a few percent. The figure given on one bike was 35% of the frontal area of an upright bike. Even at 12 mph you are starting to feel some of the effects of drag and it only goes up as you pick up even a little speed. I can tell you that when by my self I ride much faster with my high racer than with a diamond frame. Where I slow down is when my girlfriend and I ride together, she has a diamond frame 3 speed Trek cruiser and it is built to be a cruiser, not to be the pace setter in a race. At the 12-13 mph area seems to be her limit due to drag and I feel next to nothing.

No bike does everything perfectly. In fact, no bike does anything until SOMEONE gets on it and rides.

Pupsocket's observation is spot on and may be unique (the first mention I've ever seen); there is a boundary layer in any fluid flow, how thick it is and the velocity profile is beyond easy discussion (roughness, Re, et cetera).

The Lightning F40 is a faired version of the Lightning P38 SWB recumbent bicycle. I have not ridden either bike. According to the manufacturer, the Lightning F40 is supposed to be 10 MPH faster than the P38.

However, I doubt the average rider would have an F40. It's an expensive, high caliber bike--the only faired recumbent bicycle commercially available in the USA, AFAIK. I suppose that the degree to which aerodynamics benefits the average rider is the degree to which his head, his hips, and his crank line up in a straight line with the crank upstream of the hips and the hips upstream of the head.

The Lightning F40 is a faired version of the Lightning P38 SWB recumbent bicycle. I have not ridden either bike. According to the manufacturer, the Lightning F40 is supposed to be 10 MPH faster than the P38.

However, I doubt the average rider would have an F40. It's an expensive, high caliber bike--the only faired recumbent bicycle commercially available in the USA, AFAIK...

I don't know about being 10 mph faster. I know a rider who rode an unfaired P-38 on Black Bear and one that rode an F-40, and their times work out to about 5 mph faster for the F-40. BUT WAIT!!! The F-40 rider is faster all-around anyway. So the speed advantage is somewhat less. As far as being the only faired recumbent sold in the US, I think you'd have to count a socked TourEasy in the same class. HPRA does.

This year, the F-40 rider is planning on using his Quest velomobile. Which is to say, he thinks it will be faster than the F-40.

The Lightning F40 is a faired version of the Lightning P38 SWB recumbent bicycle. I have not ridden either bike. According to the manufacturer, the Lightning F40 is supposed to be 10 MPH faster than the P38.

You'll have a hard time getting objective information about the aero factor.

Not really hard to do. Find a decent hill about an 1/8 mile long at least. Measure speed and roll out destance, coasting only, with and without the fairing on a windless day.

I did this with a homemade fairing. There was a significant difference in roll out distance, but not enough to convince me a fairing was in my future. The negatives of a fairing overrode any interest I had in speed.

I will admit that the aero advantages of a fairing are certainly magnified by the mileage. This is especially true when touring. Hmm...I should rethink this. Nah. I like the wind on my body. One with the environment. All that BS. And minimal fiddle factor.

The bicycle is one of the great inventions of mankind. Delights children, challenges young men to feats of daring, and turns old men into boys again.--Me

Not really hard to do. Find a decent hill about an 1/8 mile long at least. Measure speed and roll out destance, coasting only, with and without the fairing on a windless day.

Right, you can do that if you already own a particular bike but the OP was asking about the aero benefit of a potential purchase. It's not currently possible to get reliable comparative data on different recumbent models that you don't own.

Fairing for Rans Stratus

You are forgetting something; it is the airspeed, not the ground speed that makes the difference. I have ridden my Rans Stratus for about a decade, and somewhere between 12K and 14K miles, all with the fairing. Let's say my average speed is 12 mph; that's 12 mph on the ground. But what happens when the wind is at 25-35 mph (I've actually ridden in higher winds too). You need to add that to your 12 mph, and you get 37-47 mph air speed. A fairing will definitely help you in these conditions. Also, if you are going downwind, you can add significantly to your ground speed with a "push" from the wind from behind (I'm talking about a regular fairing, and not a total body sock). I've also found my Stratus stable in high cross winds, as the bike is close to the ground and therefore a slight lean will suffice to check a sudden gust. On a regular bike, winds cause real problems that are not apparent on a faired recumbant. One more thing...I only get wet even in hard rain from the chest up--that is important in Western Oregon.
John

PS, I added a merged photo to give you an idea of what that railroad bridge, which has now been closed, looks like. From what I heard, it was used in some Western movies in the 1970s.

So, to address the original question - at ground wind speeds greater than 13-14 mph, aerodynamics matter more than mechanical efficiencies.
Recumbents use different muscles than diamond frame bikes. It takes a while to realize increased speed potential. Pedaling technique matters a great deal.
I ride uprights and bents, and love all of them.
Jon in Phily

My Son and I were riding our coupled Hase Kettwiesel the other evening and we were fighting against the headwind. Don't know how it would have felt on the DF at the same speed but was surprised that there would be that much effect on a recumbent at slower speed. Will have to compare sometime when I have the GPS with me and can ride at the same speed on both.

I know this doesn't help with your choice of recumbent your looking at but riding my trike while touring I find being low to the ground much of the side road grass or small cuttings/embankments give me a very good wind break. I dare say even on a standard two wheel bent that same environment would also kick in shielding you more so than a DF rider so it would have to have a small effect.